The technology of the disclosure relates to improving isolation between uplink and downlink channels at a wireless transceiver.
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile voice and data communication. Information is embedded in an electromagnetic signal generally within the radio frequency range of the electromagnetic spectrum. This electromagnetic signal is transmitted from a transmitter, through a first antenna, across any intervening space, to a second receiver through a second antenna. In many instances the transmitter is actually a first transceiver and the receiver is actually a second transceiver and signals are exchanged bi-directionally. Depending on point of view, one such signal may be considered an uplink signal, and the other signal may be considered a downlink signal.
In the early days of wireless communication there was generally perceived to be ample room within the electromagnetic spectrum for many such signals to coexist without interference between signals. As the complexity of the signals has increased, in part due to the increasing amounts of information placed into the signals, the electromagnetic spectrum has become relatively crowded, particularly in the radio frequency range. Accordingly, uplink signals are typically relatively close in frequency to downlink signals for bi-directional communication.
To help isolate uplink signals from downlink signals at a transceiver, there are currently a variety of solutions. One such solution is a time-based approach (e.g., time division multiplexing) that prevents simultaneous use of uplink frequencies and downlink frequencies. While effective, this approach has fallen out of favor as more information is sent in each direction making simultaneous use of the frequencies almost a requirement. Another solution is the use of a duplexer that provides isolation between uplink and downlink frequencies. An exemplary conventional transceiver 100 using a duplexer is illustrated in FIG. 1.
In particular, the transceiver 100 includes a transmit (Tx) path 102 where a signal 104 to be transmitted enters a field programmable gate array (FPGA) circuit 106 for processing and is passed through a digital-to-analog converter (DAC) 108 and an amplifier 110 to a duplexer 112. From the duplexer 112, the converted, amplified signal 104 is passed to an antenna 116 and transmitted. The transceiver 100 further includes a receive (Rx) path 118. Signals are received at the antenna 116, passed through the duplexer 112, through an amplifier 120 and an analog-to-digital (ADC) converter 122 to the FPGA 106 for processing to become a receive signal 124.
At its simplest, a duplexer is a device that allows bi-directional (i.e., duplex) communication over a single path. In the transceiver 100, the duplexer 112 isolates the receiver portion from the transmitter portion while permitting them to share a common antenna. In radio frequency communication, transmit and receive signals typically occupy different frequency bands, and so the duplexer 112 may have frequency selective filters. Modern communication often uses nearby frequency bands, so the frequency separation between transmit and receive signals is relatively small.
While duplexers may be effective at providing desired isolation, as the frequencies get closer and, particularly in the frequencies of interest, the cost of such duplexers has increased to levels that are not commercially practical. For example, such elements may cost around sixty to ninety U.S. dollars. For high frequency broadband duplexers, that cost may readily exceed one hundred U.S. dollars, and in some cases exceed three hundred U.S. dollars. Such costs are generally perceived to be unacceptable within most commercial industries.
A third solution is the use of interference cancellation calculations that may be performed in an FPGA circuit cooperating with a multiple input/multiple output (MIMO) antenna array such as illustrated in FIG. 2. In particular, a transceiver 200 may include an FPGA 202 that controls four antennas 204(1)-204(4). Antennas 204(1)-204(2) handle MIMO stream A, and antennas 204(3)-204(4) handle MIMO stream B. For MIMO stream A, the antenna 204(1) acts as the antenna for a transmit path 206(1), and the antenna 204(2) acts as the antenna for a receive path 206(2). The transmit path 206(1) includes a DAC 208 and an amplifier 210. Similarly, the receive path 206(2) has an ADC 212 and an amplifier 214. Further, a tap 216 is associated with the antenna 204(1) and provides a signal to an interference cancelation circuit 218 within the FPGA 202. Likewise, for MIMO stream B, the antenna 204(3) acts as the antenna for a transmit path 220(1), and the antenna 204(4) acts as the antenna for a receive path 220(2). The transmit path 220(1) includes a DAC 222 and an amplifier 224. Similarly, the receive path 220(2) has an ADC 226 and an amplifier 228. Further, a tap 230 is associated with the antenna 204(3) and provides a signal to an interference cancelation circuit 232 within the FPGA 202. The interference cancelation circuits 218 and 232 operate to subtract or otherwise remove the transmit signal from the received signal of the respective paths. This subtraction is done because the signal emanating from the transmit antennas 204(1), 204(3) may be received at the receive antennas 204(2), 204(4). By subtracting such received signals, the original received signal may be restored.
MIMO antenna arrays necessarily physically space antennas from one another to help isolate signals. While effective, as the number of antennas increases, the size penalty that the physical separation requires becomes impractical. This complexity is exacerbated when there is a dual band requirement for the MIMO antenna array, such as may occur in a distributed communication system (e.g., a centralized radio access network (cRAN) or distributed antenna system (DAS)). Such a situation is illustrated in FIG. 3, where a cRAN 300 has a digital routing unit (DRU) 302 coupled to a low band baseband unit (BBU) 304 and a high band BBU 306. Both the low band BBU 304 and the high band BBU 306 handle (at least) two data streams (MIMO A and MIMO B) each having an uplink (UL) and a downlink (DL) component. Thus, for the low band BBU 304, there is data stream DL MIMO A, which goes from the low band BBU 304, through the DRU 302 to a low band section 308(1) of a transceiver system 310. As with the transceiver 200 of FIG. 2, the DL MIMO A goes to an FPGA 312 and is sent through a DAC 314 and an amplifier 316 before transmission from an antenna 318. UL signals are received at an antenna 320, provided to an amplifier 322, converted in an ADC 324, and then provided to the FPGA 312. The UL signals are then passed to the low band BBU 304. Similarly, the DL MIMO B goes to the FPGA 312 and is sent through a DAC 326 and an amplifier 328 before transmission from an antenna 330. UL signals are received at an antenna 332, provided to an amplifier 334, converted in an ADC 336, and then provided to the FPGA 312. The UL signals are then passed to the low band BBU 304. As noted by signals I1 and I2, the signals transmitted from the antenna 318 may impinge on the antennas 320 and 332. Likewise, the signals 13 and 14 from the antenna 330 may impinge on the antennas 320 and 332. To reduce this interference, there is a tap 338 that collects information about the signal being sent from the antenna 318 and a tap 340 that collects information about the signal being sent from the antenna 330. These taps 338 and 340 feed interference cancelation circuits 342 and 344, respectively, which calculate an offset in a fashion similar to the transceiver 200, albeit taking into account both possible interfering signals.
With continued reference to FIG. 3, for the high band BBU 306, the DL MIMO A goes to an FPGA 346 in high band section 308(2) of the transceiver system 310 and is sent through a DAC 348 and an amplifier 350 before transmission from an antenna 352. UL signals are received at an antenna 354, provided to an amplifier 356, converted in an ADC 358, and then provided to the FPGA 346. The UL signals are then passed to the high band BBU 306. Similarly, the DL MIMO B goes to the FPGA 346 and is sent through a DAC 360 and an amplifier 362 before transmission from an antenna 364. UL signals are received at an antenna 366, provided to an amplifier 368, converted in an ADC 370, and then provided to the FPGA 346. The UL signals are then passed to the high band BBU 306. As noted by signals 15 and 16, the signals transmitted from the antenna 352 may impinge on the antennas 354 and 366. Likewise, the signals 17 and 18 from the antenna 364 may impinge on the antennas 354 and 366. To reduce this interference, there is a tap 372 that collects information about the signal being sent from the antenna 352 and a tap 374 that collects information about the signal being sent from the antenna 364. These taps 372 and 374 feed interference cancelation circuits 376 and 378, respectively, which calculate an offset in a fashion similar to the transceiver 200, albeit taking into account both possible interfering signals.
It should be appreciated that the calculations done by the interference cancelation circuits 342, 344, 376, and 378 may become more complex as the MIMO array expands past two bands or more than two antenna pairs. This complexity may add latency or otherwise impact performance and ultimately may become impractical as a solution.
Various industries have wrestled with the problem of signal isolation, but one industry that is seeing heavier use, and thus beginning to direct more attention to this issue, is in, as alluded to in the discussion of FIG. 3, distributed communication systems. An exemplary distributed communication system may be a distributed antenna system (DAS) within a building that provides wireless connections to mobile terminals within the building in places where an outside signal may be blocked or where traffic dictates that a small cell may be appropriate.
One approach to deploying a wireless communication system involves the use of radio frequency (RF) antenna coverage areas, also referred to as “antenna coverage areas.” Antenna coverage areas can have a radius in the range from a few meters up to twenty meters as an example. Combining a number of access point devices creates an array of antenna coverage areas. Because the antenna coverage areas each cover small areas, there are typically only a few users (clients) per antenna coverage area. This allows for minimizing the amount of RF bandwidth shared among the wireless system users. It may be desirable to employ optical fiber to distribute communication signals. Benefits of optical fiber include increased bandwidth.
One type of distributed antenna system for creating antenna coverage areas includes distribution of RF communication signals over an electrical conductor medium, such as coaxial cable or twisted pair wiring. Another type of distributed antenna system for creating antenna coverage areas, called “Radio-over-Fiber,” or “RoF,” utilizes RF communication signals sent over optical fibers. Both types of systems can include head-end equipment coupled to a plurality of remote units (RUs), which may include an antenna and may be referred to as a remote antenna unit or RAU. Each RU provides antenna coverage areas. The RUs can each include RF transceivers coupled to an antenna to transmit RF communication signals wirelessly, wherein the RUs are coupled to the head-end equipment via the communication medium. The RF transceivers in the RUs are transparent to the RF communication signals. The antennas in the RUs also receive RF signals (i.e., electromagnetic radiation) from clients in the antenna coverage area. The RF signals are then sent over the communication medium to the head-end equipment. In optical fiber or RoF distributed antenna systems, the RUs convert incoming optical RF signals from an optical fiber downlink to electrical RF signals via optical-to-electrical (O-E) converters, which are then passed to the RF transceiver. The RUs also convert received electrical RF communication signals from clients via the antennas to optical RF communication signals via electrical-to-optical (E-O) converters. The optical RF signals are then sent over an optical fiber uplink to the head-end equipment.
While some RUs are simple antennas that merely bring an existing cellular-type service into an area with poor reception (e.g., inside large buildings), other RUs may be more robust and may actually act as a fully functional cell (e.g., a picocell, femtocell, microcell, or the like) with registration, hand-off, and other traditional cellular functions. Still other RUs may act as some form of hybrid with some, but not all functions of a traditional cell, but more functionality than a simple antenna.
An exemplary distributed communication system is provided with reference to FIG. 4 to provide additional context. In this regard, FIG. 4 illustrates distribution of communication services to remote coverage areas 400(1)-400(N) of a wireless distribution system (WDS, also referred to herein as a distributed communication system, a distributed antenna system (DAS), or wireless communication system (WCS)) 402, wherein ‘N’ is the number of remote coverage areas. These communication services can include cellular services, wireless services, such as RF identification (RFID) tracking, Wireless Fidelity (WiFi), local area network (LAN), and wireless LAN (WLAN), wireless solutions (Bluetooth, WiFi Global Positioning System (GPS) signal-based, and others) for location-based services, and combinations thereof, as examples. The remote coverage areas 400(1)-400(N) are created by, and centered on, remote units 404(1)-404(N) (sometimes these may be low power remote units (LPR), but are more commonly referred to herein as just a remote unit (RU) or remote antenna unit (RAU)) connected to a central unit 406 (e.g., a digital routing unit (DRU) a head-end controller, a head-end unit (HEU), or the like). The central unit 406 may be communicatively coupled to a signal source 408, for example, a base transceiver station (BTS) or a baseband unit (BBU). The communicative coupling may be wireless (e.g., such as through a cellular network) or over a wire-based/fiber-based system (e.g., such as through some form of telephony network backbone or the Internet). When the signal source 408 is a BBU, the signal source 408 may communicate with the central unit 406, which may be a DRU, using digital communication protocols such as the common public radio interface (CPRI). In this regard, the central unit 406 receives downlink communication signals 410D from the signal source 408 to be distributed to the remote units 404(1)-404(N). The remote units 404(1)-404(N) are configured to receive the downlink communication signals 410D from the central unit 406 over a communication medium 412 to be distributed to the respective remote coverage areas 400(1)-400(N) of the remote units 404(1)-404(N). In a non-limiting example, the communication medium 412 may be a wired communication medium, a wireless communication medium, or an optical fiber-based communication medium. While wireless is possible, exemplary aspects of the present disclosure are well-suited for situations where the medium is a physical conductor (electrical, optical, or some other waveguide (e.g., a wireless microwave system may still use a microwave waveguide)). Each of the remote units 404(1)-404(N) may include an RF transmitter/receiver (not shown) and a respective antenna 414(1)-414(N) operably connected to the RF transmitter/receiver to distribute wirelessly the communication services to client devices 416 within the respective remote coverage areas 400(1)-400(N). The remote units 404(1)-404(N) are also configured to receive uplink communication signals 410U from the client devices 416 in the respective remote coverage areas 400(1)-400(N) to be distributed to the signal source 408. The size of each of the remote coverage areas 400(1)-400(N) is determined by an amount of RF power transmitted by the respective remote units 404(1)-404(N), receiver sensitivity, antenna gain, and RF environment, as well as by RF transmitter/receiver sensitivity of the client devices 416. The client devices 416 usually have a fixed maximum RF receiver sensitivity, so that the above-mentioned properties of the remote units 404(1)-404(N) mainly determine the size of the respective remote coverage areas 400(1)-400(N).
With reference to FIG. 4, the central unit 406 may include electronic processing devices, for example an FPGA, a digital signal processor (DSP), and/or a central processing unit (CPU), for processing the downlink communication signals 410D and the uplink communication signals 410U. Likewise, each of the remote units 404(1)-404(N) also employs electronic processing devices for processing the downlink communication signals 410D and the uplink communication signals 410U. Further, the communication medium 412 is only able to carry the downlink communication signals 410D and the uplink communication signals 410U up to a maximum bandwidth. Collectively, the processing capabilities of the electronic processing devices in the central unit 406, the processing capabilities of the electronic processing devices in the remote units 404(1)-404(N), and the maximum bandwidth of the communication medium 412 provide the system resources available in the WDS 402. In practice, the remote units 404(1)-404(N) and the client devices 416 may each have a transceiver with frequency isolation concerns.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinency of any cited documents.